The DNA damage response (DDR) has been proposed to be a barrier to tumorigenesis, such that preneoplastic lesions show activated DDR checkpoints that impede cell proliferation and induce cell senescence and/or apoptosis. Paradoxically, DNA damage checkpoints have been proposed to be important for cancer cell survival due to the extent of DNA damage incurred by these cells, either endogenously or by DNA-targeting therapy, and the need for repair to recover from damage. It was recently proposed that defects in degradation machinery for the Chk1 kinase that signals S/G2 checkpoints, could result in loss of cancer cell sensitivity to certain anticancer drugs. Interestingly, FHIT, a replicative stress target, is frequently altered in several types of cancer, alterations resulting in the loss of Fhit protein expression. Furthermore, re-expression of Fhit in cancer cells has been shown to modulate the DDR checkpoints and sensitize cancer cells to DNA damage. Our recent findings suggest that Fhit-deficient cancer cells exhibit an aberrant DNA damage checkpoint response and prolonged S-phase Chk1 activation while exposed to DNA replication stress. These observations imply a previously unknown role of Fhit as a participant in signaling Chk1 deactivation in cells with sustained replication fork block and/or extensive DNA damage. Based on these findings, we hypothesize that Fhit-directed Chk1 termination sensitizes cancer cells to DNA-targeting anticancer drugs, and that loss of Fhit expression in tumors is characteristic of cells that are unresponsive to genotoxic agents and insensitive to DNA damage. This proposed tumor suppressive function of Fhit serves to eliminate cancer cells with accumulated DNA damage, and therefore would likely contribute to the DDR tumorigenesis barrier. The goals of this project are to test these hypotheses, define the mechanism(s) of Fhit-directed Chk1 termination and assess the in vivo and clinical relevance of these findings. To this end, we will examine the mechanism of Fhit modulation of Chk1 signaling using H1299 lung cancer and MKN74 stomach cancer cell lines. We hypothesize that Fhit promotes the ubiquitylation and degradation of pChk1. Next, we will confirm our mechanistic findings in a panel of cancer cell lines and determine the biological consequence of Fhit-deficiency and failed deactivation of Chk1 signaling in cancer cells. Lastly, we will examine a TMA of 300 Triple-Negative breast cancers for loss of Fhit expression and activation of the Chk1 pathway. This TMA will be linked to clinical features, such as disease-free survival, allowing for association of absent Fhit expression with clinical outcome. Results to be obtained will provide insight into the function of Fhit as a tumor suppressor, mechanisms of failed Chk1 deactivation and defects that contribute to cancer cell progression and survival of therapy. Importantly, the results will also help to define rationales for pre-clinical studies testing Chk1 and Parp1 inhibitors in combination with chemotherapy drugs for treatment of certain types of cancer, as well as criteria for identifying patients who will benefit from such therapies. PUBLIC HEALTH RELEVANCE: All cells have DNA repair systems and 'checkpoints' to make sure the cell does not survive if there has been too much damage, but cancer cells have over-activated 'checkpoints' that allow them to survive and grow even when they have not repaired all DNA damage. Loss of expression of the Fhit tumor suppressor protein enhances the ability of cancer cells to survive extensive DNA damage, even the damage caused by chemotherapy. This project will define the mechanism through which Fhit modulates the 'checkpoint', thus contributing to the understanding of aberrant 'checkpoints in cancer cells.